This invention pertains to loudspeakers in general and, in particular, to economical loudspeakers for high fidelity reproduction which do not require conventional speaker enclosures. More specifically, this invention pertains to variably tuned air column loudspeaker systems.
It is well known that loudspeakers radiate sound from both the front and rear of the speaker cone, and that the air in front of the cone is compressed and simultaneously the air in the rear of the cone is rarefied, when the speaker cone is pushing air in front of it. Since sound compressions and rarefactions are 180 degrees out of phase with each other, the sound in front of the cone is out of phase with that radiated from the rear; therefore, when these two out-of-phase sound waves meet, they cancel or short circuit each other. This cancellation affects primarily the low-frequency sound, since high frequency sound is radiated in the form of beams, which do not meet.
In the past, the obvious answer to this situation was to put some sort of obstacle around the speaker to prevent the front and rear sound waves from reaching each other. These obstacles have generally been in the form of baffles and were designed with the theory that the longer the path length from front to rear compared to the wavelength of the sound to be radiated, the less cancellation would take place. Several types of baffles evolved from this theory beginning with the flat open baffle, which mostly comprised a large flat wooden board.
Following the flat open baffle, came the open back enclosure, the infinite baffle, the bass-reflex enclosure, and the labyrinth enclosure. Each of these baffles added to and improved the quality of loudspeaker systems, particularly in the low frequency range.
The infinite baffle, which was basically a sealed box, had undesirable qualities which made it impractical. It wasted the sound output from the rear of the speaker cone; it had undesirable damping effects on the speaker, and it was a very large box that required a great space. However, the inherent defects of the infinite baffle were overcome by a revolutionary principle of speaker and baffle design known as acoustic suspension. The elastic suspensions of the conventional loudspeakers cannot reproduce the large excursions demanded by low bass frequencies, without considerable distortion. Acoustic suspension avoids this difficulty by vastly increasing the "compliance" of the mechanical spring suspensions and then compensating up for the lost spring action through a pneumatic spring consisting of the compressed air in the sealed box. The speaker is acoustically suspended in the infinite baffle, which is calculated to provide the proper amount of pneumatic spring action through its sealed-in air. The pneumatic spring action is linear, hence distortion free.
An alternative, earlier approach consisted of providing an additional opening, or air vent, in the front of an infinite baffle, whereby the sound energy from the rear of the speaker can be used. This was called the bass-reflex enclosure, and has become one of the most popular hi-fi baffles. The bass-reflex enclosure is essentially a phase inverter for low frequency sound waves. By making the length of the acoustic path from the rear of the speaker to the opening just right by proper tuning of its enclosure, the rear wave can be delayed sufficiently so that it emerges from the opening in phase with the front wave and thus reinforces it. As a result of this reinforcing action, the low-frequency output at certain frequencies can be twice that of an infinite baffle.
Labyrinth speakers are described in an article entitled, "Labyrinth Speakers for Hi-Fi", by David B. Weims, and appearing in the January 1972 issue of Popular Electronics. As stated therein, the labyrinth speaker is a tuned pipe or tube with an open end, into one end of which is mounted a transducer or speaker. When the wave from the speaker reaches the other remote end of the pipe, it spreads out into the listening environment, causing a sudden pressure drop which reflects back through the pipe to the speaker as a rarefaction. At the quarter-wave frequency (.lambda./4) of the sound, the air in the mouth of the pipe is at minimum velocity, but maximum pressure (Bernoulli's Theorem). This condition produces an accompanying, maximum rarefaction reflected back to the speaker cone. Hence, the anti-resonant action of the pipe or tubular enclosure offers maximum damping to the speaker if the length of the pipe is adjusted to the quarter-wave frequency of the sound produced by the speaker.
At the frequency at which the length of the pipe is a half-wave (.lambda./2) of the sound, the air in the mouth of the tubular enclosure is at low pressure, but high velocity. Because there is no sudden change in pressure as the wave moves out of the pipe or tubular enclosure, there is no anti-resonance action by the pipe, and the speaker cone is able to move freely, and because of the .lambda./2 phase shift (180) within the tubular enclosure, the emerging wave is in-phase with that coming from the front of the speaker, adding to the speaker's output. In the noted article, there are disclosed various configurations by which the desired labyrinth enclosure may be formed, all of which are characterized as being necessarily rigid enclosures, either of the type having a series of walls protruding within an essentially rectangular enclosure or of a pipe-type enclosure, wherein both the effective length of the labyrinth is said to be approximately one-fourth wavelength of the natural resonance frequency of the speaker, which is inserted into the labyrinth enclosure. Similar labyrinth disclosures also are disclosed by U.S. Pat. Nos. 3,443,660, 3,523,589 and 3,687,220 of the inventor of this invention and assigned to the assignee of this invention; likewise, they are characterized by disclosing a relatively rigid enclosure.
In the prior art design labyrinth speakers, it is well-known to determine the length of the labyrinth speaker to be approximately one-fourth wavelength of the sound reproduced. As noted at page 44 of the above-identified article, "The performance of a straight open pipe can be predicted, but for a pipe that is both folded and stuffed, one needs a crystal ball". Further, it is desired to design the speaker enclosure to make it aesthetically appealing as a piece of furniture, and therefore it is not always desirable to incorporate a straight, rigid pipe of the desired wavelength, which might otherwise adversely affect the furniture design of this type of enclosure. In addition, the natural resonant frequency f.sub.o of the speaker incorporated into such an enclosure is not readily controlled by the manufacturer, and only in the most expensive speakers is the natural resonant wavelength predictable from speaker to speaker. Thus, in minimizing the overall cost of the speaker system, it would be desirable to use relatively low-cost speakers, the resonant frequency wavelengths of which may vary considerably, as much as .+-.20% from speaker to speaker. Therefore, it is highly desirable to provide some type of adjustment to compensate for varying speakers to be enclosed that otherwise would be impossible with a rigid speaker enclosure.